Induction heating of metal by a high frequency current is widely being used for quenching and other heat treatment. It is being used as a heating system for replacing indirect heating by conventional gas heating and electric heating with the object of controlling the material quality of steel plate, aluminum plate, and other ferrous and nonferrous thin-gauge plate during the production process and, further, with the object of increasing the heating speed to improve productivity and freely adjust production volume, etc.
There are generally two systems for induction heating a metal plate. One is an induction heating system called the “LF (longitudinal flux heating) system” running a high frequency current through an induction coil surrounding the circumference of a metal plate to generate flux which passes through the metal plate in the longitudinal direction and generates an induction current inside the cross-section of the metal plate to heat it. The other is the “TF (transverse flux heating) system” arranging a metal plate between good magnetic bodies called inductors around which primary coils are wound, running current through the primary coils to generate flux, and passing these through the inductors so as to make the flux running between the inductors cut across the metal plate transversely and thereby generate an induction current at the plane of the metal plate and induction heating it.
The LF system of induction heating provides good temperature distribution uniformity, but the generated induction current circulates inside the plate cross-section. Due to the relationship of the current penetration depth, when the plate is thin, it is necessary to raise the frequency of the power source or else no induction current will be produced. Further, with a nonmagnetic material or even a magnetic material in which Curie point temperature is exceeded, the current penetration depth will become deeper, so there is the problem that thin plate cannot be heated.
On the other hand, the TF system of induction heating has the flux pass through the plane of the metal plate, so is characterized by being capable of heating irrespective of thickness or being magnetic or nonmagnetic and is characterized by using inductors with low magnetic resistance and thereby being capable of reducing the leakage flux and by being capable of concentrating the flux between the inductors facing the front and back of the metal plate, so having a high heat efficiency.
On the reverse side, there is the problem that uneven temperature distribution is liable to occur and the problem that when the metal plate is not at the center of the facing inductors, a magnetic material will be drawn to one of the inductors and greater temperature deviation will more easily be caused. Further, there is the defect that in the case of the TF system of induction heating, it is difficult to handle changes in width of the metal plate and snaking in continuous processing lines.
To solve these problems, Japanese Patent Publication (A) No. 2002-43042 discloses arranging single-turn coils of the front surface and back surface of a strip in the direction of progression offset in the direction of progression. Further, Japanese Patent Publication (A) No. 2002-151245 proposes a diamond-shaped induction coil where the long axis of the induction heating coil facing the heated material curves in the width direction of the heated material. Further, in Japanese Patent Publication (A) No. 2005-209608, the inventors proposes shifting an induction coil circling a metal strip in the width direction in the direction of progression at the front surface side and back surface side of the metal plate.